As shown in FIG. 14, a steering device for an automobile is configured to transmit rotation of a steering wheel 1 to an input shaft 3 of a steering gear unit 2, and to push and pull a pair of left and right tie rods 4, 4 by rotation of the input shaft 3, thereby applying a steering angle to front wheels. The steering wheel 1 is supported and fixed to a rear end portion of a steering shaft 5, and the steering shaft 5 is rotatably supported to a cylindrical steering column 6 with being axially inserted into the steering column 6. A front end portion of the steering shaft 5 is connected to a rear end portion of an intermediate shaft 8 via a universal joint 7, and a front end portion of the intermediate shaft 8 is connected to the input shaft 3 via another universal joint 9.
In the above-described steering device, there has been known an electric power steering device configured to reduce a force necessary to operate the steering wheel 1 by using an electric motor 10 as an auxiliary power source. The electric power steering device is incorporated with a reducer. As such reducer, there has been used a worm reducer having a large lead angle and reversibility with respect to a transmission direction of power. FIG. 15 shows a structure disclosed in Patent Document 1 as an example of the conventional structure of the worm reducer incorporated in the electric power steering device. A worm reducer 21 is provided within a reducer housing 11 fixed to an electric motor 10a and has a worm 14 having worm teeth 13 formed on an axially intermediate portion of a worm shaft 12 and a worm wheel 15 configured to mesh with the worm teeth 13. Both axial end portions of the worm 14 are rotatably supported in a worm accommodation part 17 of the housing 11 by a pair of rolling bearings 16a, 16b such as deep groove ball bearings. One end portion (a left end portion in FIG. 15) of the worm 14 is connected to an output shaft 18 of the electric motor 10a, so that the worm 14 can be rotated.
The worm wheel 15 is rotatably provided within a wheel accommodation part 19 of the housing 11, and a center of rotation axis thereof is arranged at a skew position with respect to the worm 14. The worm wheel 15 is made by fixing a tooth part 20 of a synthetic resin around a metallic wheel part 29 so as to save the manufacturing cost and the weight. The tooth part 20 is meshed with the worm teeth 13 without a gap (i.e. backlash is zero), so that a rotational force between the worm wheel 15 and the worm 14 can be transmitted without causing a time lag resulting from the backlash. The worm wheel 15 is externally fitted to a portion close to a front end of a steering shaft 5a. Thereby, it is possible to transmit the rotation driving force generated from the electric motor 10a to the steering shaft 5a via the worm reducer 21 having the worm 14 and the worth wheel 15.
The worm reducer 21 is assembled as follows, for example. First, an outer ring of the tip-side rolling bearing 16a (right, in FIG. 15) is internally fitted (internally fitted by interference fit) to a bearing holding part 22a provided at an inner end portion (a right end portion, in FIG. 15) of the worm accommodation part 17 of the housing 11 which is configured to accommodate therein the worm 14. Then, the worm wheel 15 is rotatably supported within the wheel accommodation part 19 of the housing 11. Then, the worm 14 is inserted into the worm accommodation part 17 through an opening of the worm accommodation part 17 at a state where the worm 14 is offset outward (upward, in FIG. 15) in a radial direction of the worm wheel 15 with respect to a center axis of the worm accommodation part 17 so that the worm teeth 13 do not interfere with the tooth part. 20 of the worm wheel 15. At a stage where the worm 14 is inserted into the worm accommodation part 17 to some extent, the worm 14 is displaced inward (downward, in FIG. 15) in the radial direction of the worm wheel 15 to mesh the worm teeth 13 and the tooth part 20 of the worm wheel 15 each other. With the worm teeth 13 and the tooth part 20 being meshed with each other, the worm wheel 15 is rotated to further insert the worm 14 into the worm accommodation part 17, so that a tip portion of the worm 14 is press-fitted into an inner ring of the tip-side rolling bearing 16a. Then, the base-side rolling bearing 16b (left, in FIG. 15) is press-fitted between a bearing holding part 22b provided at an intermediate portion of the worm accommodation part 17 and an outer periphery of an intermediate portion (the base side portion than the worm teeth 13) of the worm 14 (an outer ring of the base-side rolling bearing 16b is internally fitted to the bearing holding part 22b by loose fit or interference fit, and an inner ring thereof is externally fitted to the outer periphery of the intermediate portion of the worm 14 by interference fit). Subsequently, a snap ring 24 is engaged with an engagement groove 23 provided on an inner periphery of the worm accommodation part 17, so that axial displacements of the base-side rolling bearing 16b and the worm 14 are restricted. Then, the output shaft 18 of the electric motor 10a is connected to a base end portion (a left end portion, in FIG. 15) of the worm 14, and the opening of the worm accommodation part 17 is closed by a motor case 25 having the electric motor 10a accommodated therein.
According to the above method of assembling the worm reducer, the worm 14 and the pair of rolling bearings 16a, 16b for rotatably supporting the worm 14 are separately mounted in the worm accommodation part 17 of the housing 11. Therefore, the assembling cost increases, and the manufacturing cost of the worm reducer 21 increases.
In Patent Document 2, a worm shaft having each ball bearing attached thereto is inserted into a worm shaft accommodation part of the housing having a preload applying member attached thereto, and after the insertion, each ball bearing is applied with preload by an elastic restoring force of an arm part of the preload applying member. In this case, since the worm shaft is inserted while the outer ring of the base-side ball bearing is brought into contact with the arm part of the preload applying member, the worm shaft is necessarily required to move in the axial direction. Therefore, the worm wheel needs to be assembled after the assembling of the worm shaft so as not to interfere with the tip-side ball bearing at the assembling of the worm shaft.